1 Technical Field of the Invention
The present invention relates generally to a current-measuring circuit designed to measure the current flowing through an electric path, and more particularly to a current-measuring circuit which has a simple structure, yet is capable of current measurement with high accuracy.
2 Background of Related Art
U.S. Pat. No. 5,081,379 to Korteling teaches a current-sensing circuit designed to measure the current flowing through a power transistor.
FIG. 1 shows a current-sensing circuit like the one taught in Korteling which includes an current-carrying transistor Q1, a current-measuring transistor Q2, and n-channel MOS transistors Q4 and Q5. The current-carrying transistor Q1 is an n-channel MOS transistor which is connected at a drain to a voltage source VD and at a source to ground through an electric load L. The current-measuring transistor Q2 is an n-channel MOS transistor which is connected at a drain to the drain of the current-carrying transistor Q1 and at a gate to a gate of the current-carrying transistor Q1. The MOS transistor Q4 has a drain and a source disposed in a current path extending from the source of the current-measuring transistor Q2 to ground and has a gate connected to the drain thereof. The MOS transistor Q5 constitutes a current mirror together with the MOS transistor Q4.
The current-sensing circuit also includes an operational amplifier OP and a p-channel MOS transistor Q3. The operational amplifier OP is connected at a non-inverting input (+) to the source of the current-carrying transistor Q1 and an inverting input (-) to the source of the current-measuring transistor Q2 and serves to match the source voltage of the current-measuring transistor Q2 to the source voltage of the current-carrying transistor Q1. The MOS transistor Q3 is disposed in series between the source of the current-measuring transistor Q2 and the drain of the MOS transistor Q4 and connected at a gate to an output terminal of the operational amplifier OP.
When a common voltage is applied to the gates of the current-carrying transistor Q1 and the current-measuring transistor Q2, the current flows the transistors Q1 and Q2, thereby causing the drain-source voltage of the MOS transistor Q3 driven by an output of the operational amplifier OP to change, so that the source voltage of the current-measuring transistor Q2 agrees with the source voltage of the current-carrying transistor Q1. This causes electric potentials appearing among all the terminals of the current-carrying transistor Q1 and the current-measuring transistors Q2 to be identical with each other. The current I.sub.Q1, thus, flows through the current-carrying transistor Q1 (i.e., the load current flowing through the electric load L), while the current I.sub.Q2 which is determined by a size ratio of the current-measuring transistor Q2 to the current-carrying transistor Q1 flows through the current-measuring transistor Q2 and enters the MOS transistor Q4 through the MOS transistor Q3. This causes the current i that is a multiple of the current I.sub.Q2 to flow through the MOS transistor Q5, which is used in measuring the current I.sub.Q1 flowing through the current-carrying transistor Q1.
As apparent from the above discussion, the conventional current-sensing circuit is capable of measuring the current I.sub.Q1 flowing through the current-carrying transistor Q1 accurately by matching the source voltage of the current-measuring transistor Q2 to the source voltage of the current-carrying transistor Q1 with aid of activities of the operational amplifier OP and the voltage-controlling MOS transistor Q3, however, it has the following drawbacks.
The agreement of the source voltage of the current-measuring transistor Q2 with that of the current-carrying transistor Q1 is achieved by controlling the gate-source voltage of the voltage-controlling MOS transistor Q3. The source of the MOS transistor Q3 is connected to the source of the current-measuring transistor Q2, and the source voltage of the current-measuring transistor Q2 is basically equal to the source voltage that is an output voltage Va of the current-carrying transistor Q1, therefore, the operational amplifier OP needs to change the voltage Vb outputted to the gate of the MOS transistor Q3 depending upon the output voltage Va of the current-carrying transistor Q1.
Specifically, the operational amplifier OP is required to provide a voltage output over a range within which the output voltage Va of the current-carrying transistor Q1 will change. For instance, when the output voltage Va of the current-carrying transistor Q1 changes from 1 volt or less to several tens of volts, the operational amplifier OP needs to change the voltage Vb outputted to the gate of the MOS transistor Q3 greatly according to the change in output voltage Va.
Accordingly, the conventional current-sensing circuit needs to consider the output voltage Va of the current-carrying transistor Q1 in determining the voltage and capacity of a power source of the operational amplifier OP, thus resulting in complexity of circuit design.